Add information to the app template for new-comers

- link to the meta data reference
- add all categories
- add help with the build configuration

templates/app-full

@@ -1,13 +1,31 @@
 # F-Droid metadata template
 #
 # See http://f-droid.org/manual for more details
+# and the Metadata reference
+# https://f-droid.org/docs/Build_Metadata_Reference/
 #
 # Fields that are commented out are optional
 #
 # Single-line fields start right after the colon (with a whitespace).
 
-Categories:
- - First
+Categories: (use those which apply)
+ - Connectivity
+ - Development
+ - Games
+ - Graphics
+ - Internet
+ - Money
+ - Multimedia
+ - Navigation
+ - Phone & SMS
+ - Reading
+ - Science & Education
+ - Security
+ - Sports & Health
+ - System
+ - Theming
+ - Time
+ - Writing
 License: (identifier from https://spdx.org/licenses)
 
 # Website: (web link)
@@ -59,6 +77,8 @@ Builds:
 #     Here go the notes to take into account for future updates, builds, etc.
 #     Will be published in the wiki if present.
 
+# The following options are described at this location:
+# https://f-droid.org/docs/Build_Metadata_Reference/#UpdateCheckMode
 AutoUpdateMode: (see the manual)
 UpdateCheckMode: (see the manual)
 CurrentVersion: (current version name)